JP5654680B2 - Energy saving in heavy hydrocarbon distillation. - Google Patents

Description

This application claims priority from US patent application Ser. No. 12 / 868,286, filed Aug. 25, 2010.
The present invention relates to an improved method of energy saving in the distillation of hydrocarbons. More specifically, the present invention relates to energy saving in a complex facility for aromatic compound processing that produces xylene isomers.

Xylene isomers are produced in large quantities from petroleum as a feedstock for various important industrial chemicals. The most important of the xylene isomers is para-xylene, which is the primary feedstock for polyester and continues to enjoy high growth rates due to the large volume of basic demand. Ortho-xylene is used to produce phthalic anhydride and is produced in large quantities but is supplied to a relatively mature market. Metaxylene is used in products such as plasticizers, azo dyes, and wood preservatives, although the amount used is less than that. Ethylbenzene generally is present in xylene mixtures, although sometimes recovered for styrene production, generally believed to be less desirable components in the C ₈ aromatics.

Among aromatic hydrocarbons, the overall importance of xylene as a feedstock for industrial chemicals is comparable to benzene. Xylene and benzene are produced from petroleum by naphtha reforming, but are insufficient to meet demand, and therefore conversion of other hydrocarbons is necessary to increase xylene and benzene production It is. Often toluene produces the C ₈ aromatics either generate benzene is dealkylated or the selective disproportionation by benzene and the xylene isomers are recovered.

  The Aromatic Compound Complex diagram is disclosed by Meyers in the Handbook of Petroleum Refining Processes, 2d. Edition, 1997, McGraw-Hill, hereby incorporated by reference. Incorporated into.

  Aromatic complex facilities that produce xylene consume considerable energy, particularly in distillation operations where feedstock is prepared to separate the product from the conversion process. The separation of xylene from heavy aromatic compounds shows great potential for energy savings. Energy savings in such processes not only reduce processing costs, but also address current concerns about carbon emissions.

Broad aspect of the present invention is a process for producing para-xylene from a feed stream containing C ₈ aromatics and C ₉ and heavier aromatics. The method includes: (a) separating C ₈ aromatics and C ₉ or higher heavy aromatics contained in at least one low boiling feed stream and at least one high boiling feed stream 2 A distillation process involving two xylene towers. Wherein the at least one high boiling feed stream, high content of the at least one low boiling point above C ₉ than the feed stream of heavy aromatics. The distillation step includes distilling the at least one high-boiling feed stream at a first pressure in a first xylene column and the first C ₈ aromatics stream to a first C ₉ or higher heavy aroma. Separating from the group compound stream and distilling the at least one low-boiling feed stream in the second xylene column at a second pressure, and the second C ₈ aromatics stream into the second C ₉ Separating from the above heavy aromatics stream. The second pressure is higher than the first pressure, and the overhead stream from the second xylene tower exchanges heat with the reboiler of the first xylene tower. (B) the C ₈ injected into either or both, as well as the desorbent stream adsorption step aromatics stream, a first mixture containing paraxylene and desorbent, as well as a second mixture comprising a raffinate and desorbent A para-xylene separation step of recovering para-xylene from either or both of the first and second C ₈ aromatics streams. (C) distilling the second mixture in a pre-distillation column and subsequently producing a retentate stream and a desorbent stream through the pressurized retentate tower, wherein the desorbent stream is passed to the paraxylene separation step. A desorbent recovery step of separating the second mixture of step (b) by returning. The top stream of the first xylene column in step (a) supplies heat to the reboiler of the pressurized extraction residue column.

  Further objects, aspects and details of the present invention can be obtained and inferred from the following detailed description of the invention.

FIG. 1 schematically shows an aromatic compound complex facility to which the energy saving concept can be applied. FIG. 2 shows an aromatic compound complex facility to which energy saving is applied. FIG. 2A shows another embodiment relating to the xylene column and the residual liquid column. FIG. 2B shows another aspect associated with the residual liquid column. Figure 3 shows the application of energy saving when distilling the C ₈ aromatics from heavy aromatics. FIG. 4 shows an example of a specific device in an aromatic compound complex where energy saving is achieved by direct heat exchange. FIG. 5 shows an aromatic compound complex where some of the energy saving concepts described herein are applied as a supplement or alternative to other energy savings. FIG. 6 shows the generation of steam from a particular device in the aromatic compound complex.

The feed stream to the process of the present invention is usually of the general formula C ₆ H _(6-n) R _n , where n is an integer from 0 to 5, each R being CH ₃ , C ₂ H ₅ , any combination of C ₃ H ₇ or C ₄ H ₉ ). The aromatics-rich feed stream to the process of the present invention may be derived from a variety of feedstock sources, including but not limited to light olefins and heavy aromatics-rich byproducts (often pyrolysis gasoline Naphtha, distillate or other hydrocarbons that produce gasoline range feedstocks and steam pyrolysis, and distillate and heavy oils that produce gasoline range products or Includes pyrolysis and the like. Products obtained from pyrolysis or other cracking operations remove sulfur, olefins and other compounds that can adversely affect product quality and / or damage the catalyst or adsorbent used. For this purpose, hydroprocessing is generally performed according to methods well known in the art prior to filling the complex. In addition, light cycle oil obtained from catalytic cracking is conveniently hydrotreated and / or hydrocracked according to well-known techniques to obtain gasoline range products, preferably hydrotreated to aromatic compounds. It may include catalytic reforming to obtain a rich feed stream. When the feed stream is a catalytic reforming oil, the reformer is preferably very demanding to keep the concentration of non-aromatic compounds in the product low and increase the yield of aromatics. It is operated by.

FIG. 1 is a simplified flow diagram of a typical aromatic compound processing complex of well-known technology aimed at producing at least one xylene isomer. The complex facility may, for example, process a feedstock rich in aromatic compounds obtained from catalytic reforming. Typically, such streams are treated to remove olefinic compounds and light fractions (eg, butane and lighter hydrocarbons, and preferably pentane). However, this removal is not essential to the practice of the broad aspects of the present invention. Aromatic compound-containing feed stream, benzene, toluene, containing a C ₈ aromatics typically contain aliphatic hydrocarbons containing higher aromatics and naphthenes than that.

The feed stream contains C ₈ and heavier aromatics taken through conduit 10 and through heat exchanger 12 to reformed oil separator 14 where it is distilled and removed as a bottoms stream in conduit 16. A stream is separated from toluene and lighter hydrocarbons recovered as overhead distillate via conduit 18. Toluene and lighter hydrocarbons are sent to the extractive distillation processor 20, and the primarily aliphatic residue in conduit 21 is separated from the benzene-toluene aromatics stream in conduit 22. The aromatics stream in conduit 22 is passed through benzene column 23 in conduit 24 along with the stripped transalkylation product in conduit 45 and the overhead distillate from the paraxylene finishing tower in conduit 57. Separated into a benzene stream and a heavy aromatics stream above toluene in conduit 25 that is sent to toluene tower 26. Toluene is recovered from this column as a top distillate in the conduit 27, and a part or all of the toluene may be sent to the transalkylation apparatus 40 described later.

The bottom stream from the toluene tower 26 passes through the conduit 28 along with the bottom oil from the reformer separator in the conduit 16 after being treated in the clay processor 17 and the recycled C ₈ aromatics in the conduit 65. It is sent to the rectification tower 30. The rectification column 30 contains C ₈ aromatics concentrated as a top distillate in conduit 31 and C ₉ , C ₁₀ and heavier aromatics as a bottoms flow in conduit 32. Separate from the boiling stream. This bottom stream is sent to the heavy tower 70 through a conduit 32. The heavy aromatic column provides a top stream containing C ₉ and at least some C ₁₀ aromatics in conduit 71, while higher higher grades, mainly C ₁₁ or higher, which are higher boiling compounds. The alkylaromatic compound is withdrawn as a bottom stream via conduit 72.

C ₉ + aromatics from the heavy column in conduit 71 are combined with the toluene-containing top distillate contained in conduit 27 as feed to transalkylation reactor 40. The transalkylation reactor contains a transalkylation catalyst well known in the art that produces a transalkylation reactant comprising benzene along with the desired xylene from C ₁₁ + aromatics. The transalkylation product in conduit 41 is the C ₆ hydrocarbons and lighter returned via conduit 44 to extractive distillation column 20 for recovery of the gas in conduit 43 and light aromatics and purification of benzene. Stripping in stripper 42 to remove any hydrocarbons. The bottom oil from the stripper is sent to the benzene tower 23 through a conduit 45, and the benzene product and unconverted toluene are recovered.

The C ₈ aromatic compound top distillate supplied by the rectification column 30 contains para-xylene, meta-xylene, ortho-xylene and ethylbenzene, and is sent to the para-xylene separation step 50 via the conduit 31. The separation step preferably serves as follows by adsorption using a desorbent. A mixture of paraxylene and desorbent is fed via conduit 51 to extraction tower 52 where paraxylene passing through conduit 53 is separated from the recovered adsorbent in conduit 54. The para-xylene is purified in the finishing tower 55 to produce a para-xylene product that passes through the conduit 56 and a light feed that is returned to the benzene tower 23 via the conduit 57. Non-equilibrium mixture of C ₈ aromatic raffinate and desorbent from the separation step 50 is sent to the raffinate column 59 via conduit 58 where the raffinate for isomerization in conduit 60 is a conduit Separated from the recovered desorbent in 61.

The retentate containing a non-equilibrium mixture of xylene isomers and ethylbenzene is sent to isomerization reactor 62 via conduit 60. Raffinate but is isomerized in the reactor 62, the reactor includes a isomerization catalyst to provide the product close to the equilibrium concentration of C ₈ aromatics isomers. The product is sent via conduit 63 to deheptane tower 64 where C ₇ hydrocarbons and lighter hydrocarbons are removed, and the bottom oil is sent via conduit 65 to xylene tower 30 where C ₉ and raw materials for heavy is separated from the C ₈ aromatics which are isomerized than. Overhead liquid from the de-heptane tower 64 is sent to a stripper 66, where, from the raw material of C ₆ and C ₇ are sent via 68 conduit extractive distillation apparatus 20 for the recovery of valuable benzene and toluene, the conduit 67 The light material at the top of the tower is removed.

As those skilled in the art will appreciate, many variations of this scheme are possible within the skill of the art. For example, it may be subjected to only the portion containing the entire C ₆ -C ₈ reformate or benzene extraction. Para-xylene can also be recovered from the C ₈ aromatic compound mixture by crystallization rather than adsorption. Like para-xylene, meta-xylene can be recovered from the C ₈ aromatic compound mixture by adsorption, and ortho-xylene can be recovered by fractional distillation. Alternatively, aromatic C ₉ and heavier stream or heavy aromatics stream, which is processed using a stripping or other means by solvent extraction or solvent distillation or steam with a polar solvent, highly concentrated as retentate stream Group compounds are separated from C ₉ + recycles that are transalkylated. In some cases, the entire heavy aromatics stream may be processed directly in a transalkylation unit. The present invention is useful in these aromatic treatment schemes and other variations, aspects of which are described in US Pat. No. 6,740,788, incorporated herein by reference.

For the separation of the C ₈ aromatic compound from the heavy aromatic compound in the rectifying column 30, the distillation step of the present invention is usually effective. The distillation process of the present invention comprises a C ₈ aromatic compound and a C ₉ + aroma contained in a plurality of internal or external feed streams respectively designated as first and second feed streams of an aromatic compound complex. Represented by a plurality of xylene towers, each carrying out separation from a group compound in substantially the same manner. Preferably, the two streams comprise a first feed stream having a high boiling point and a second feed stream having a low boiling point, wherein the content of C ₉ + hydrocarbons in the first feed stream having a high boiling point is Higher than the second feed stream. The present invention is separated from at least one of the first heavy aromatics stream at fractionation column by distilling first feed stream at low pressure the first C ₈ aromatics stream over the first C ₉ Separating the second C ₈ aromatics stream from the second C ₉ or higher heavy aromatics stream by distilling the second feed stream at high pressure in a second fractionation tower; and Circulating the top stream from the second column to supply heat to the reboiler of the first column. The low pressure is typically between 100 and 800 kPa, and the high pressure is chosen to allow heat transfer from the second column to the first column, typically at least 400 kPa higher than the low pressure. . The concept of different pressures in parallel towers is the case where the heavy components present in the high-boiling feed stream are decomposed at the reboiler temperature required to separate the light and heavy components. Especially valuable.

The second fractionation tower has a second feed stream in which the concentration of heavy feed to be decomposed is lower than the feed stream to the first tower, and therefore heat exchange between the first and second towers. By doing so, the pressure can be increased in order to save energy without lowering the yield of the product or contaminating the apparatus. The feedstock is preferably not only contains most or all of the isomerization C ₈ aromatics from the isomerization reactor after removal heptane of other C containing heavy aromatic compound having a low concentration _Eight aromatics streams may be included. This stream to the second column is typically less than 10% C ₉ + aromatics, often less than 5% C ₉ + aromatics, and more often less than 2% C ₉ + Includes aromatic compounds. Effectively, the process involves heating the top distillate to the reboiler of the first column, and preferably to at least one other column, and / or to the steam generator of the associated processing complex. Including operating the second column at a pressure that allows it to be fed.

In another aspect, the process includes operating the second fractionation column at a pressure that allows the top distillate to supply heat that generates steam that is useful in the associated processing complex. Furthermore, C ₈ aromatics fractionator may comprise three or more columns comprising additional heat exchange between overheads and reboilers in a manner similar to that described above.

  FIG. 2 is an energy efficient aromatic compound complex using many of the concepts of the present invention. A parallel numbering system is employed in FIGS. 1 and 2 for ease of reference. The feed stream is routed through conduit 110, through heat exchangers 112 and 113, where the temperature is raised and sent to the reformer separator 114. As will be explained later in this section, heat exchange takes place from the net paraxylene product and the recovered paraxylene separation process recovered desorbent via conduit 213 and conduit 214, respectively.

As in FIG. 1, C ₈ and heavier aromatics are withdrawn as a bottom stream in conduit 116, while toluene and lighter hydrocarbons recovered from the top of the column via conduit 118. Is fed to an extractive distillation processor 120 that separates primarily the aliphatic residue in conduit 121 from the benzene-toluene aromatics stream in conduit 122. The aromatics stream in conduit 122 is combined with the stripped transalkylation product in conduit 144 and the overhead distillate from the paraxylene finishing column in conduit 157 in fractionator 123 and in conduit 124. And the heavy aromatics stream above toluene in the conduit 125 sent to the toluene column 126. Toluene may be recovered from this column at the top of the column via conduit 127 and sent partially or wholly to the transalkylation unit 140 as described below.

The bottoms stream from the toluene column 126 via conduit 128, along with recycled C ₈ aromatics in the bottoms oil and conduit 148 from reformate separator in conduit 116 after being processed by the clay treater 117, It is sent to the low pressure xylene tower 130. Other C8 aromatic streams, including streams obtained from sources outside the complex and having a significant content of C9 and heavier aromatics, may also be treated in this column and are removed from the stream 165. Part of the bottom oil of the heptane tower may also be included depending on the overall energy balance. The low pressure xylene column separates concentrated C ₈ aromatics as the top distillate in conduit 131 from the high boiling stream containing C ₉ , C ₁₀ and heavier aromatics as the bottom stream in conduit 132. To do.

At the same time, the isomerized C ₈ aromatics stream is sent via conduit 165 to the high pressure second xylene column 133. This is characterized as a low boiling feed stream in which the concentration of the heavy matter to be decomposed is lower than the feed to the column 130, and thus the column pressure can be increased to save energy. Containing stream obtained from a source of complex external, likewise C ₉ and heavier aromatics other C ₈ aromatics containing flow low content also may be included in feed to this column . The second xylene column separates the second C ₈ aromatics stream as the top distillate in conduit 134 from the second C ₉ or higher heavy aromatics in conduit 132. At least a portion of the overhead vapor from the high pressure xylene tower in conduit 134 is preferably used in the reboiler 135 for reboiling of the low pressure xylene tower 130 and passes through conduit 136 to the xylene separation step 150 as condensed liquid. Sent to the column 133 and refluxed (not shown). Also, the overhead distillate in conduit 134 is preferably used to provide energy to the reboiler of the extraction column 152 and other equipment described below or obvious to those skilled in the art.

The C ₉ + bottom stream sent to the reboiler 137 is used to transfer energy to the conduit before the reboiler to reboiler either the heavy aromatics column 170 and / or the extraction residue column 159, respectively. Supply is made through either or both of the flow in 270 and the heated flow in conduit 259 from the reboiler, and the bottom stream after the heat exchange is sent to the heavy aromatic compound tower 170. Other similar heat exchange facilities will be apparent to those skilled in the art. The net bottom stream in conduit 138 typically passes through tower 130, but may directly join the flow in conduit 132 to heavy tower 170 through conduit 139. The heavy column provides a top stream in conduit 171 containing C ₉ and at least some C ₁₀ aromatics, while higher alkyl aromatics, primarily C _{11 and} higher, are in conduit 172 as the bottom stream. It is taken out through. This column may be reboiled by the bottom oil of the xylene column in conduit 270 as described above. The top steam from tower 130 and tower 170 may also generate steam via conduit 230 and conduit 271 respectively, as shown, and the condensed liquid is either as reflux to each tower or the net of stream 131 or stream 171 respectively. It functions as a top distillate.

C ₉ + aromatics from the heavy column in conduit 171 are combined with the toluene-containing top distillate contained in conduit 127 as feed to transalkyl reactor 140 to produce a transalkyl containing xylene. The product is formed. The transalkylation product in conduit 141 is stripped by stripper 142 to remove the gas in conduit 143 and C ₇ and lighter liquids, and C ₇ and lighter liquids are light aromatics. Is recovered by the isomerization stripper 166 through the conduit 144 and then returned to the extractive distillation column 120. The bottom oil from the stripper is sent to the benzene tower 123 via a conduit 145, and the benzene product and unconverted toluene are recovered.

Paraxylene, meta-xylene, ortho-xylene, and the first and second C ₈ aromatics stream supplied by the xylene column 130 and 133 containing ethylbenzene, through conduits 131 and 136 of xylene - isomer separation step 150 Sent. Although the description herein is applicable to the recovery of one or more xylene isomers other than paraxylene, a description of paraxylene is presented for ease of understanding. The xylene isomers are separated by an adsorption process, and the desired isomer recovered using a desorbent is extracted. The adsorbent can be used in the form of a fixed bed in alternating contact with the feed stream and desorbent, or it can be used in multiple beds with appropriate piping / valves and the feed stream can be connected to one or more adsorbent beds It may be passed continuously while the desorbent stream passes through the other adsorbent beds in the set. In the counter-flow moving bed system operation, a steady concentration profile of the feed mixture components is achieved and the introduction of the feed and desorbent streams is operated at a fixed point along with the extraction and extraction of the retentate. It can be carried out. In the simulated moving bed method, the progressive movement of fluid through the plurality of access points along the adsorption chamber assumes the movement of the adsorbent contained in one or more chambers. The operating principle and process sequence of the simulated moving bed flow system is described in U.S. Pat. Nos. 2,985,589, 3,310,486, and 4,385,993. Is incorporated herein by reference.

  In the adsorption step, a first mixture of paraxylene and desorbent is supplied to extraction tower 152 via conduit 151 where the paraxylene via conduit 153 is separated from the recovered desorbent in conduit 154. Extraction column 152 is preferably at high pressure so that the top distillate from the column is at a temperature sufficient to reboil finishing column 155 via conduit 256 or deheptane column 164 via conduit 265. , At least 300 kPa, more preferably 500 kPa or more. Either by heat supplied to reboil the load via conduit 256 and conduit 265, either refluxed to column 152 (not shown) or sent to finishing column 155 as a net flow in conduit 153. Condensation of the extract, or both, occurs in these streams. Para-xylene is purified in finishing tower 155 to produce a para-xylene product via conduit 156 and a light feed that is returned to benzene tower 123 via conduit 157.

A second mixture of the retentate and desorbent as a non-equilibrium mixture of C ₈ aromatics from separation step 150 is sent via conduit 158 to the retentate column 159 where it is isomerized in conduit 160. The retentate for is separated from the recovered desorbent in conduit 161. The extraction tower can be operated at high pressure and produce steam via conduit 260 or heat exchange in other areas of the complex, and the liquid condensed by such heat exchange can be extracted from the extraction liquid. Used as reflux to the column or as a net overhead extract in conduit 160. The recovered desorbent in line 154 and line 161 and the net bottom tower bottom oil can heat the feedstock in line 110 flowing in via line 213 and line 212, respectively. Energy saving can be achieved by adjusting the devices and operations around the extraction liquid tower 159 as shown in FIGS. 2A and 2B. These figures illustrate improvements to selected portions of FIG.

  FIG. 2A shows the energy savings due to the pressurization of the extraction liquid column under the condition of avoiding decomposition of the desorbent in the column reboiler. By making the pressurized distillation column normally operated above 400 kPa, preferably above 500 kPa, the top stream of the column generates steam and / or other components in the aromatic compound complex. While it is possible to exchange useful energy with the stream, such pressurization results in the tower bottom temperature being a temperature that causes significant desorbent decomposition. FIG. 2A shows a portion of the complex facility of FIG. 2 associated with the lower portion of the high pressure xylene column 133 and the extracted residue column 159. Here, at least a portion of the effluent from reboiler 237 comes from the liquid in conduit 241 that is returned to column 133 along with all the rest of the reboiler effluent in conduit 238. Is sent to a drum 240 for separating the steam. The steam in conduit 242 provides heat to reboiler 243 and maintains a peak membrane temperature below the decomposition limit, preferably using an exchanger having a high nucleate boiling surface, as described below. The overhead condenser preferably operates at a temperature that produces medium pressure steam and produces net non-equilibrium mixed xylene in conduit 160.

  FIG. 2B shows another energy saving method focusing on the region of the residual liquid column 159 of FIG. The feedstock mixture of the retentate and desorbent in conduit 158 is sent to preliminary rectification column 258 where a portion of the net mixed xylene stream that is the top distillate through condenser 261 is condensed into conduit 262 It is collected via. The remaining xylene and desorbent are sent via conduit 264 to pressurized extraction residue column 159. The overhead distillate is at least partially condensed via pre-rectification column reboiler 263 via conduit 265 and returned to the top of the distillation column via conduit 266 and the remainder of the mixed xylene via conduit 160. Collected. As shown in FIG. 2, this column is reboiled with the overhead distillate in conduit 259 from the high pressure xylene column, and the net desorbent is returned to its previous adsorption state via conduit 161.

The retentate containing the non-equilibrium mixture of xylene isomers and ethylbenzene is sent to isomerization reactor 162 via conduit 160. In the isomerization reactor 162, raffinate is isomerized product approaching the equilibrium concentration of C ₈ aromatic isomers are fed. The product is sent via conduit 163 to deheptane tower 164 where C ₇ hydrocarbons and lighter hydrocarbons are removed, preferably recycled by the overheads in conduit 265 from extraction tower 152. Boiled. Bottoms oil from de-heptane tower is sent to a xylene column 133 through conduit 165, C ₉ and heavier material than it is separated from the isomerized C ₈ aromatics. The overhead liquid from the deheptane tower 164 is sent to a stripper 166, where the light product of the overhead distillate in the conduit 167 passes through a conduit 168 for extraction and distillation of the benzene and toluene values for recovery and purification Separated from the C ₆ and C ₇ feeds sent to 120. The pressure in the deheptane tower 164 and stripper 166 is selected for heat exchange or generation of steam in a manner similar to the xylene tower described elsewhere herein.

FIG. 3 shows further details of the heat exchange of the present invention between parallel xylene distillation columns 130 and 133. The feed to the low pressure xylene tower 130 is bottom oil from the toluene tower via conduit 128, clay treated bottom oil from the reformer separator via conduit 116, purge C ₈ aromatics in conduit 138. It includes a compound, if appropriate in the C ₈ aromatics containing streams and energy balance of the other that is not suitable for processing in the high pressure xylene column may comprise a portion of the de-heptane stream 165. The mixed feedstock of the heavy reformed oil and the bottom oil of the toluene tower may contain heavy aromatic compounds that are easily decomposed at high temperatures, and by operating at a pressure lower than 800 kPa, the bottom of the tower and The reboiler can maintain a temperature that avoids such decomposition. The low pressure xylene column is from a high boiling stream containing concentrated C ₈ aromatics as the top distillate in conduit 131 and C ₉ , C ₁₀ and heavier aromatics as the bottom stream in conduit 132. To separate. The top stream from column 130 may be used at least in part via conduit 230 of FIG. 2 to generate steam as described above, or to reboil other columns, and is therefore condensed to the column. Or a net overhead distillate for separating xylene in the conduit 131.

At the same time, the isomerized C ₈ aromatics stream is sent via conduit 165 to the high pressure xylene tower 133, which contains a lower concentration of heavy feed to be cracked than feed to tower 130. In order to save energy by simultaneously using a high temperature that can be used when heat exchange is effectively performed, the pressure of this column is increased from the pressure of the low-pressure xylene column according to the present invention as described above. Accordingly, the temperature of the overhead vapor from the high pressure xylene tower 133 is sufficient to supply useful energy to other equipment within the aromatic compound complex. As described above, the temperature of the top vapor is sufficient to reboil the low pressure xylene column 130 of the reboiler 135, providing reflux to the column 133 and a net flow in the conduit 136. Bottom stream of a small amount of net in conduit 138 is fed to the lower pressure column 130 for recovery of C ₈ aromatics remaining.

  Alternatively or in addition, the temperature of the overhead vapor from the high pressure xylene tower 133 is sufficient to generate steam useful for equipment heating or to reboil the tower in other processing equipment. Such steam is generally generated at pressures above 300 kPa, preferably at 500 kPa, and most preferably at 1000 kPa or higher. The overhead stream may be indirectly heat exchanged with a water circuit that supplies the steam drum. Most commonly, boiler feed water is heated with a heat exchanger disconnected from the steam drum. A plurality of water circuits serving various exchangers are arranged in parallel with each other and fed to a single steam drum to provide the desired pressure steam product requiring only one set of metering devices. Such steam systems are well known and details are added, such as by the teachings found in US Pat. No. 7,730,854, which is incorporated herein by reference in its entirety.

  Energy recovery according to the present invention, often involving closer temperatures between process fluids, is improved by the use of heat exchangers having surfaces with high nucleate boiling capabilities. Such high nucleate boiling surfaces include, for example, U.S. Pat. Nos. 3,384,154, 3,821,018, 4,064,914, 4,060,125, 3, 906,604, 4,216,826; 3,454,081, 4,769,511, and 5,091,075, all of which are incorporated herein by reference. Can be achieved in various ways. Such high flow piping is particularly suitable for heat exchange between the second high pressure xylene tower overhead and the reboiler of the first low pressure xylene tower or the generation of steam from the top of the xylene tower. Yes.

  Typically, these high nucleate boiling surfaces were coalesced to the surface of a shell-and-tube heat exchanger tube. These improved tubes can be made in a variety of ways well known to those skilled in the art. For example, such a tube may include an annular or helical cavity that extends along the tube surface made by machining the tube. Alternatively, fins may be provided on the surface. Further, a cut may be made in the tube to provide a ridge, a groove, a porous layer, and the like.

  In general, more efficient modified tubes are those having a porous layer on the boiling side of the tube. The porous layer can be provided by many different methods well known to those skilled in the art. The most efficient of these porous surfaces have what are referred to as recessed cavities that confine vapor within the layer cavities by confined cavity openings. In one such method, a porous boiling layer is bonded to one side of a thermally conductive wall, as described in US Pat. No. 4,064,914. An essential feature of the porous surface layer is capillary-sized pores that are interconnected, some of which communicate with the outer surface. The liquid to be boiled enters the subsurface cavity through external holes and subsurface interconnect holes and is heated by the metal that forms the walls of the cavity. At least a portion of the liquid is vaporized within the cavity, so that bubbles grow on the walls of the cavity. A portion of it eventually exits the cavity through the outer surface holes, then rises through the liquid film covering the porous membrane and divides into the gas space above the liquid film. Further, liquid flows from the interconnect hole into the cavity, and this mechanism is repeated continuously. Such an improved tube containing a porous boiling layer is commercially available under the registered trademark High Flux Tubing from UOP, Des Plaines, IL.

  FIG. 4 shows that the overhead distillate from one or more hot towers can achieve energy savings by directly exchanging heat with the reboiler of one or more cold towers. An example of a specific device is shown using the numerical representation of the process of FIG. The overhead distillate in conduit 134 from the high pressure xylene column 133 has a temperature sufficient to supply energy for reboiling the extraction column 152 via the reboiler 235 and is either reflux or net. The xylene tower top distillate to be returned to 133 as a column distillate is condensed in conduit 236. The extraction tower is such that the top extract in conduit 256 has a temperature sufficient to reboil finishing tower 155 (which preferably operates at vacuum pressure) via reboiler 257. Pressurize to condense the extract column overheads in conduit 258. As described above, para-xylene product is recovered in conduit 156.

  FIG. 5 summarizes some of the direct heat exchange possibilities associated with FIG. 2, but is not exhaustive or excludes others. The high-pressure xylene tower 133 can supply heat for reboiling one or more of the low-pressure xylene tower 130, the extraction tower 152, and the extraction liquid tower 159. The low pressure xylene tower 130 can supply heat to reboil the extractive distillation tower 120. Pressurized extraction column 152 can provide heat to reboil one or more of benzene column 123 and finishing column 155. The pressurized extraction liquid tower 159 can supply heat to reboil one or more of the reformed oil separator 114, the toluene tower 126 and the deheptane tower 164.

FIG. 6 summarizes examples of the possibility of indirect heat exchange by the generation of intermediate pressure steam, but is not exhaustive. The overhead stream 230 (FIG. 2) from the low-pressure xylene tower 130 and the overhead stream 260 (FIG. 2) from the pressurized residual tower 159 are 0.6-2 MPa, preferably 0.7, in the header 100. ~ 1.5 MPa of medium pressure steam can be generated, and this steam is used to reboil one or more of the reformer separator 114, extractive distillation unit 120, and toluene column 126, the other There is a further possibility of sending steam to the device. Such steam generation and use can be considered as a supplement or substitute for other energy savings as described in FIG. For example, the high pressure xylene tower 133 supplies heat to reboil the low pressure xylene tower 130 and the extraction tower 152, which then reboils the benzene tower 123 and finishing tower 155.
Example The combination of steam generation and direct heat exchange described above with reference to FIG. 6 was evaluated from the viewpoint of investment recovery. The reference case is the facility shown in FIG. 1, and the case of the present invention is the case of FIG. 6 applied to the system diagram of FIG. The relative important parameters for the production of para-xylene are as follows:

Reference Example The present invention Fuel consumption 1.0 0.922
Net consumption steam 1.0 0
Net generated steam 1.0 1.35

Claims (10)

A method for producing para-xylene from a feedstock containing C ₈ aromatics and C ₉ and heavier aromatics,
(A) a distillation step comprising two xylene towers separating C ₈ aromatics contained in at least one low boiling feed stream and at least one high boiling feed stream from C ₉ or higher heavy aromatics; There,
The content of at least one high boiling feed stream said at least one low boiling point above C ₉ than the feed stream of heavy aromatics are many,
The distillation step includes distilling the at least one high-boiling feed stream at a first pressure in a first xylene column and the first C ₈ aromatics stream to a first C ₉ or higher heavy aroma. Separating from the group compound stream and distilling the at least one low-boiling feed stream in the second xylene column at a second pressure, and the second C ₈ aromatics stream into the second C ₉ Separating from the heavy aromatics stream, wherein the second pressure is higher than the first pressure, and the overhead stream from the second xylene column is reboiling of the first xylene column. A distillation process for exchanging heat with the vessel;
(B) the C ₈ injected into either or both, as well as the desorbent stream adsorption step aromatics stream, a first mixture containing paraxylene and desorbent, as well as a second mixture comprising a raffinate and desorbent by obtaining a paraxylene separation process for recovering paraxylene from either or both of the first and second C ₈ aromatics stream,
(C) Distilling the second mixture in a pre-distillation column and subsequently passing through a pressurized extraction residue column to produce an extraction residue stream and a desorbent stream, wherein the desorbent stream is separated from the paraxylene. A desorbent recovery step for separating the second mixture in step (b) by returning to the step, wherein the bottom stream of the second xylene column in step (a) is the pressurized residue column A desorbent recovery process for supplying heat to the reboiler,
Including methods.   The method of claim 1, wherein the second pressure is at least 400 kPa higher than the first pressure.   The method according to claim 1, wherein the first pressure is between 100 kPa and 800 kPa.   The method according to any one of claims 1 to 3, wherein the reboiler of the first xylene column has a surface with a high nucleate boiling capacity. It said at least one high boiling feed stream exceeds 5 wt% C ₉ and heavier aromatics, the method according to any one of claims 1 to 4. Wherein at least one low-boiling feed stream is less C ₉ or more than 5 wt% heavy aromatics, the method according to any one of claims 1 to 5. Wherein at least one low-boiling feed stream comprises a part or the whole of the bottom stream from those de heptane C ₈ aromatics isomerization products The method of claim 6.   The method according to any one of claims 1 to 7, wherein a tower top stream from the pressurized remnant tower exchanges heat with a steam generator to generate medium pressure steam.   The heat exchange between the overhead stream from the second xylene tower and the reboiler of the first xylene tower condenses the overhead stream from a second xylene tower. 9. A method according to any one of claims 8.   The method according to any one of claims 1 to 9, wherein the para-xylene separation step is an adsorption separation step of a simulated moving bed.

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